In brief:
Depends

Some are extremely serious and cause death, like anencephaly, where most of the brain is missing. Some are minor and and easy to treat, like an extra finger. Different chemicals are known to be associated with different kinds of birth defects, so it completely depends on which product or chemical you are using.

In brief:
Depends

Some are extremely serious and cause death, like anencephaly, where most of the brain is missing. Some are minor and and easy to treat, like an extra finger. Different chemicals are known to be associated with different kinds of birth defects, so it completely depends on which product or chemical you are using.
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It also depends upon the stage of pregnancy that you are in. Early in pregnancy compounds can cause birth defects, but after the body is formed compounds can no longer effect the structure of the body part, only its function.